In a known rear wheel steering vehicle, an actuator is provided between a link mechanism that determines the steering angle and the vehicle body, and the steering angles of the right and left wheels are individually varied depending on the cornering condition of the vehicle by extending and retracting the actuator so as to change the dimension between the vehicle body and link mechanism. See Japanese patent laid open publication (kokai) No. 09-030438 (patent document 1). Such a rear wheel steering vehicle may include devices for stabilizing the motion stability of the vehicle such as a ABS (anti-lock braking system) for preventing the locking of wheels when braking, TCS (traction control system) for preventing the free miming of wheels when accelerating the vehicle and VSA (vehicle stability assist syStem) that is additionally provided with a function to limit a side slipping of the vehicle when cornering.
In connection with rear wheel steering vehicles equipped with a vehicle motion stability control system, it is known to prohibit the control action of the vehicle motion stability control system when the rear wheel steering device has become faulty and is thereby rendered incapable of performing a normal rear wheel steering. See Japanese patent laid open publication (kokai) No. 09-123889 (patent document 2).
In the rear wheel steering vehicle disclosed in patent document 1, upon occurrence of a failure in the rear wheel steering control device, the control by the vehicle motion stability control system is disabled. However, if the rear wheels are caused to be fixed in a certain condition (such as a toe out condition) owing to the occurrence of a failure (such as system down) of the rear wheel steering control device, the motion stability of the vehicle when traveling straight ahead or making a turn may be impaired. Therefore, if the vehicle motion stability control system is capable of operating properly, it is desirable to activate the vehicle motion stability control system upon occurrence of an abnormal condition in the rear wheel steering control device so as to achieve a motion stability of the vehicle. In particular, if the vehicle is equipped with a vehicle motion stability control system that can be selectively turned on and off, it is necessary to force the vehicle motion stability control system to be turned on when the vehicle motion stability control system is turned off.